V. E. Pavlov

Late Mesoproterozoic magnetostratigraphic results from Siberia: Paleogeographic implications and magnetic field behavior

We present a magnetostratigraphic study of the late Mesoproterozoic Malgina and Linok Formations, located along the southeastern (Uchur-Maya region) and northwestern (Turukhansk region) margins of the Siberian craton, respectively. Biostratigraphic, radiometric, and chemostratigraphic data indicate that these formations are likely coeval between 1050 and 1100 Ma. Paleomagnetic analyses reveal a high-temperature component carried by magnetite and/or hematite. This component yields positive fold and reversal tests, together with a positive conglomerate test for the Malgina Formation, which indicates that the magnetization was acquired during or soon after sediment deposition. The mean paleomagnetic direction obtained from the Uchur-Maya region, which is unambiguously representative of the Siberian craton, indicates that it could not have been part of Rodinia at that time if Siberia was located in the Southern Hemisphere and if we assume that Laurentia and Siberia were connected along their present northern shorelines. We emphasize that Siberia could have been part of Rodinia during the late Mesoproterozoic if southern Siberia was joined to the northern part of Laurentia as recently proposed by Rainbird et al. [1998]. If true, placing the Siberian craton in the Southern Hemisphere implies that the magnetic polarity of the ∼1000 Ma Laurentian paleomagnetic poles must be switched. Our data also show the occurrence of at least 15 symmetric geomagnetic field reversals, indicating that the paleomagnetic results from the late Mesoproterozoic Keweenawan lavas do not reflect a worldwide and persistent asymmetric field during the Proterozoic.

The SPICE carbon isotope excursion in Siberia: a combined study of the upper Middle Cambrian–lowermost Ordovician Kulyumbe River section, northwestern Siberian Platform

An integrated, high-resolution chemostratigraphic (C, O and Sr isotopes) and magnetostratigraphic study through the upper Middle Cambrian–lowermost Ordovician shallowmarine carbonates of the northwestern margin of the Siberian Platform is reported. The interval was analysed at the Kulyumbe section, which is exposed along the Kulyumbe River, an eastern tributary of the Enisej River. It comprises the upper Ust’-Brus, Labaz, Orakta, Kulyumbe, Ujgur and lower Iltyk formations and includes the Steptoean positive carbon isotopic excursion (SPICE) studied here in detail from upper Cambrian carbonates of the Siberian Platform for the first time. The peak of the excursion, showing δ13C positive values as high as+4.6‰and least-altered 87Sr/86Sr ratios of 0.70909, is reported herein from the Yurakhian Horizon of the Kulyumbe Formation. The stratigraphic position of the SPICE excursion does not support traditional correlation of the boundary between theOrakta and Labaz formations at the Kulyumbe River with its supposedly equivalent level in Australia, Laurentia, South China and Kazakhstan, where the Glyptagnostus stolidotus and G. reticulatus biozones are known to immediately precede the SPICE excursion and span the Middle–Upper Cambrian boundary. The Cambrian–Ordovician boundary is probably situated in the middle Nyajan Horizon of the Iltyk Formation, in which carbon isotope values show a local maximum below a decrease in the upper part of the Nyajan Horizon, attributed herein to the Tremadocian Stage. A refined magnetic polarity sequence confirms that the geomagnetic reversal frequency was very high during Middle Cambrian times at 7–10 reversals per Ma, assuming a total duration of about 10 Ma and up to 100 magnetic intervals in the Middle Cambrian. By contrast, the sequence attributed herein to the Upper Cambrian on chemostratigraphic grounds contains only 10–11 magnetic intervals.

Carbon isotope stratigraphy of the Precambrian-Cambrian Sukharikha River section, northwestern Siberian platform

A high-resolution carbon isotope profile through the uppermost Neoproterozoic-Lower Cambrian part of the Sukharikha section at the northwestern margin of the Siberian platform shows prominent secul ...

Secular Geomagnetic Variations and Volcanic Pulses in the Permian-Triassic Traps of the Norilsk and Maimecha-Kotui Provinces

Detailed paleomagnetic studies have shown that the effusive Permian-Triassic traps in the Kotui River valley were formed as the result of volcanic activity, which occurred in the form of volcanic pulses and individual eruptions with net duration of at most 7000–8000 years, excluding the periods of volcanic quiescence. According to the analysis of the paleomagnetic data earlier obtained by Heunemann and his coauthors [2004b] on the Abagalakh and Listvyanka sections in the Norilsk region, those geological units were formed during 25 volcanic pulses and separate eruptions, which all lasted up to 8000 years altogether, whereas the total time of formation (including the periods of volcanic quiescence) exceeded 10000–100000 years for the Norilsk section and was probably a bit shorter for the Kotui section. Comparison of the positions of virtual geomagnetic poles calculated for the Norilsk and the Kotui sections provides no grounds to suggest that these sections were formed at different geological times. The scatter in the positions of the virtual geomagnetic poles (VGP) for the directional groups and individual directions (58 altogether) jointly for the two sections (more than 160 lava flows) indicates that the secular geomagnetic variations at the Permian-Triassic boundary had similar amplitudes to those that occurred in the past 5 Ma.

Middle Cambrian high magnetic reversal frequency (Kulumbe River section, northwestern Siberia) and reversal behaviour during the Early Palaeozoic

We present a magnetostratigraphic study of Middle Cambrian sediments along the Kulumbe river (northwestern Siberia). The deposits are V850 m thick and lie within the Mayan stage as constrained by trilobite fossils. Palaeomagnetic analyses reveal two magnetic components. The first component, parallel to the direction of the present day field at the site, is isolated in the low to middle temperature range. A high temperature component (HTC) is then isolated up to 580 or 680‡C, carried either by magnetite and/or by hematite. The HTC defines a sequence of 28 magnetic polarity intervals, some of them being defined by only one sample. Considering that the studied section has a duration of V5 Myr, we propose that the magnetic reversal frequency was high (V4^6 reversals per Myr) during part of the Middle Cambrian, among the highest values known within the Phanerozoic. The reversal frequency may have been roughly similar during the Lower Cambrian. We further suggest a drastic decrease of the magnetic reversal frequency between the Lower^Middle Cambrian and the end of the Tremadoc (Ordovician) when a superchron probably occurred. However, this behaviour is still challenged by other scenarios which depend on the chosen Early Palaeozoic time scale and on the reliability of some magnetostratigraphic data. fl 2000 Elsevier Science B.V. All rights reserved.

Siluro-Devonian paleomagnetic results from the Tuva Terrane (southern Siberia, Russia): implications for the paleogeography of Siberia

New paleomagnetic data from seven sections of Siluro-Devonian age sedimentary sequences in the Tuva Terrane (south Siberia, Russia) reveal the presence of up to three different components of magnetization. Component A is of recent or Late Mesozoic-Cenozoic origin, and component B, identified unambiguously in 25 samples from one section, yields a sample mean direction (Declination/Inclination) of 300.6/−54.7 (k = 42.2, α95 = 4.5°) in geographic and 283.9/−63.5 (k = 45.4, α95 = 4.3°) in stratigraphic coordinates. However, since the results of the fold test are inconclusive, no further attempt has been made to assign an age to this magnetization. Component C, with unblocking temperatures of 500°–680°C, is identified in all seven sections and with dual polarity. After bedding correction, the section mean directions all plot along a common small circle in stereographic projection with a mean inclination of 42.1° (N = 7 sites, k = 184.2, α95 = 2.9°). It passes the inclination-only fold test, the within-site fold test (both on the 95% significance level) and the reversal test and is thus interpreted as being primary in origin and Siluro-Devonian in age. Adopting the normal polarity option for component C yields a paleolatitude of 24°N (±2°) for the Tuva Terrane. On the basis of geological evidence, the Tuva Terrane was clearly accreted to Siberia by early Silurian times. The results obtained in this study, therefore, indicate lower paleolatitudes for Siberia than those predicted from previously published Apparent Polar Wander Paths for the Siberian Platform and demonstrate that Siberia was positioned between the equator and 30°N during latest Silurian/earliest Devonian times. These new results are also supported by paleoecological and paleoclimatological evidence. The dispersion of declinations along a small circle is indicative of clockwise block rotations within the Tuva Terrane of up to 72° with respect to Siberia.

Magnetostratigraphy of the Ordovician Angara/Rozhkova River section: Further evidence for the Moyero reversed superchron

We present new magnetostratigraphic results obtained from a well-dated Ordovician key section located along the Angara River, near the terminus of the Rozhkova River (southern Siberian platform). More than 220 samples were thermally demagnetized up to 670°C in order to isolate their characteristic ancient magnetization. Samples from the Arenig, the Llanvirn and the Llandeilo stages are all (but two) of reversed magnetic polarity. In contrast, samples dated of the Caradoc yield a sequence of several magnetic polarity intervals. These new data therefore confirm the occurrence of a long reversed magnetic polarity interval during the Ordovician, the so-called Moyero superchron, which ended during the middle or late Llandeilo.

New paleomagnetic data on the Anabar Uplift and the Uchur-Maya region and their implications for the paleogeography and geological correlation of the Riphean of the Siberian Platform

The results of paleomagnetic studies of the reference sections of the Riphean and Late Proterozoic intrusive bodies of two remote areas of the Siberian Platform are presented. Within the limits of the Uchur-Maya region the sedimentary rocks of the Gonam, Omakhta, Ennin and Konder formations were studied; and the Riphean sedimentary rocks of the Burdur and Kotuykan formations on the northern and western slopes of the Anabar Uplift and, also, the Late Proterozoic intrusive complexes, located in the basins of the Fomich, Magan, Dzhogdzho and Kotuykan Rivers were studied. The paleomagnetic poles obtained in the course of this work and the present-day geochronological data give grounds to assume that: (1) the accumulation of the Riphean of the Anabar Uplift occurred after the formation of the Uchurskaya series of the Uchur-Maya region and was completed in approximately 1.5 Ga; (2) the Konder layers, compared according to the correlation pattern accepted at the present time [Semikhatov and Serebryakov, 1983] with the bottoms of the Totta formation, can be related to the appreciably more ancient stratigraphic level; (3) the intrusion of the studied intrusive bodies of the northern and western slopes of the Anabar Uplift occurred nonsimultaneously, although within close time intervals of approximately 1.5 Ga. The estimates of the kinematic parameters of the drift of the Siberian Platform within an interval of 1.7–1.0 Ga is carried out.

Paleomagnetic studies and estimation of geomagnetic paleointensity at the early/middle Riphean boundary in rocks of the Salmi Formation (North Ladoga area)

The territory of Karelia (Baltic Shield) is virtually not represented in the global paleomagnetic database for the Lower Riphean time interval (1650—1350 Ma). As regards the paleointensity Han, the huge interval 1–2 Ga in length is represented in the global paleointensity database by only eight determinations concentrated in the interval 1–1.35 Ga. The paper presents results of paleomagnetic studies of volcanic and subvolcanic rocks composing the Early Riphean Salmi Formation, which outcrops in the valley of the lower Tulemaioki River in the northern coast area of Lake Ladoga. Results of the study indicate that, in the Early Riphean time, the East European craton was located in the tropical region of the Southern Hemisphere between 15° S and 40° S. The inferred value of Han is close to the lower boundary of the interval (1.36–11.56) × 1022 A m2, encompassing previously published intensity values of the paleofield 1–1.35 Ga; this supports the hypothesis on the existence of long intervals of a lower field in the period in question [Maquoin et al., 2003].

Katav Limestones: A Unique Example of Remagnetization or an Ideal Recorder of the Neoproterozoic Geomagnetic Field

The Neoproterozoic Katav Formation, participating in the composition of the Urals stratotype top section, includes a magnetostratigraphic record that can be the source of unique information on geomagnetic field behavior 800–900 Ma ago. For many years, it has been considered that the characteristic magnetization of this formation is metachronous and was formed in the course of the Late Paleozoic diastrophism. New data, obtained during the performed studies, indicate that the characteristic magnetization of the formation is primary, and the formation itself is an ideal recorder of the Neoproterozoic geomagnetic field.